JP2001000417A - Magnetic resonance imaging method for heart using multiple slabs and multiple windows - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable to obtain a 3D MRI data collection in a short time by setting multiple respiration gate windows, performing a pilot scanning defining corresponding multiple slabs, and combining the obtained 3D MRI data to form a single 3D MRI data collection indicating an interesting area. SOLUTION: This system is controlled by an operator console 100 connected to a computer 107 via a link 116, and the computer 107 is connected to a system control part 122 containing a pulse generation module 121 via a high-speed serial link 115. To obtain a 3D MRI data collection, multiple respiration gate windows (hereinafter, designated as G.W) are set, and a 3D interesting area is divided into multiple 3D slabs corresponding to multiple G.Ws. When detected respiration points are in the range of multiple windows of the G.Ws, a MRI data is obtained from the corresponding 3D slabs.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The field of the invention is nuclear magnetic resonance imaging methods and systems. More specifically, the present invention relates to correction of MRI data acquired during exercise of a patient.

[0002]

BACKGROUND OF THE INVENTION When a substance, such as human tissue, is exposed to a uniform magnetic field (polarizing magnetic field B ₀ ), the individual magnetic moments of spins in the tissue tend to align along this polarizing magnetic field, Precess around the polarization field in random order at the Larmor frequency specific to each spin. When a substance or tissue is exposed to a time-varying magnetic field (excitation field B ₁ ) that is in the xy plane and has a frequency close to the Larmor frequency, the aligned net magnetic moment M _z rotates, ie, toward the xy plane. "tipped", to produce a net transverse magnetic moment M _t. A signal is emitted by the spins thus excited, and an image can be formed by receiving and processing the signal.

When forming an image using these signals, magnetic field gradients (G _x , G _y and G _z ) are used. Typically, the imaging area is scanned by a series of measurement cycles in which these gradients change depending on the particular localization method employed. The resulting set of NMR received signals is digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.

The present invention is described in detail with reference to a variant of the well-known Fourier transform (FT) imaging technique, often referred to as "spin warp". Spin warp method is used in Physics in Medicine and Biolog
y, Volume 25, pp. 751-756 (1980)
Of WA Edelstein et al. “Spin-warp NM
R imaging and application to whole body imaging (Spin-Warp NMR Imaging and Applications to Human W
hole-Body Imaging). This technique uses a phase encoding magnetic field gradient pulse having a variable amplitude before acquiring an NMR signal to phase encode spatial information in the direction of the gradient. For example, in a two-dimensional implementation (2DFT), spatial information is encoded in one direction by applying a phase encoding gradient (G _y ) in that direction, and then in a direction orthogonal to the phase encoded direction. A signal is acquired in a state where a readout magnetic field gradient (G _x ) is present in the first step. The spatial information is encoded in the orthogonal direction by the readout gradient present at the time of acquisition. In a typical 2DFT pulse sequence, in the chain of the "view" that are acquired during the scan by increasing the size of the phase-encoding gradient pulse G _y (ΔG _y), as can reconstruct the entire image NMR Form a set of data.

[0005] Most of the NMR scans currently used to produce high-resolution three-dimensional medical images, such as coronary artery images, may require several minutes to acquire the required data. Due to the long scan time, the patient's movement during the scan can be significant and the reconstructed image can be corrupted with motion artifacts. There are also many types of patient movements, such as respiratory movements, heart movements, blood flow and peristalsis. Methods used to reduce or eliminate these motion artifacts include methods that reduce motion (eg, breathing) and methods that reduce the effects of motion (eg, US Pat. No. 4,663,591). ) And methods of correcting data acquired during known movements (eg, US Pat. No. 5,200,700). In the case of respiratory movements, one of the most widely known methods of reducing motion artifacts is to reduce the data such that views are acquired only during a predetermined portion of the respiratory cycle or "acquisition window". Is a method of gate-controlling the acquisition of.

[0006] Conventional respiratory gating methods include means for detecting patient respiration (eg, US Patent No. 4,994,473) and generating a gating signal for an MRI system during a predetermined portion of the respiratory cycle. Used. As long as the gate signal is generated, the MRI system acquires NMR data in a defined view order. During other parts of the breathing exercise, the gate signal is turned off and no data is acquired.
As a result, the use of a respiratory gate greatly increases scan time since data can only be acquired over a relatively small portion of each respiratory cycle.

[0007] A method is known in which NMR data is acquired during the movement of an object to be inspected and the data is corrected instead of acquiring the NMR data in a relatively short acquisition time. These methods often use a navigator pulse sequence that is interleaved with the acquisition of NMR image data and designed to determine the location of the object to be examined. For example, US Pat. No. 5,363,844
Discloses a navigator pulse sequence that measures the position of the patient's diaphragm throughout image data acquisition. Magn. Reson. In Med., Volume 32, Volume 6
"Real-Time Motion Detection in Spiral MRI Using Navigator" by TS Sachs et al., Pp. 39-645 (1994).
This position information can be used to exclude image data acquired during portions of the respiratory or cardiac cycle that cause unacceptable image artifacts, as described in “Iral MRI Using Navigators”. Also,
Magn. Reson. In Med., Vol. 37, pp. 148-1
"Prospectively Adaptive Navator Correction for Respiratory MR Coronary Angiography (Angiography)" by MV MacConnell, p. 52 (1997).
igator Correction for Breath-hold MR Coronary Angi
ography) ”
The position information from the echo signal may be used predictively to adjust the reference phase of the receiver of the MRI system to correct subsequently acquired NMR image data. Alternatively, Proc.
International Society of Magnetic Resonance in Med
"Corinary MRA Using Navigator Echo," by ME Brummer et al., icine magazine, p. 748 (1995).
Of Respiratory Motion Artifacts
In Coronary MRA Using Navigator Echoes), the phase of the acquired k-space image data may be corrected retrospectively using the position information based on the navigator signal.

When acquiring a three-dimensional image of a moving examination object, such as a coronary artery, a series of thin slabs are excited one after the other and the NMR data acquired from each slab are concatenated. It is desirable to obtain a three-dimensional NMR data set from the region of interest. US Patent No. 5,1
No. 67,232, "Magnetic Resonance Angiography by Three-Dimensional Acquisition of Continuous Thin Thin Slabs (Magnetic Resonan
ce Angiography By Sequential Multiple Thin Slab Th
As described in “ree-Dimensional Acquisition”, these thin slabs are superimposed to prevent signal loss at boundaries due to imperfect slab excitation profiles. When using the respiration gate method, a single respiration gate window is set up and all data for each slab is acquired in the gate window. Data acquisition for the next slab does not start until data acquisition for the current (current) slab is completed.
For this reason, a long scanning time may be required.

[0009]

SUMMARY OF THE INVENTION The present invention is a method and system for acquiring multi-slab 3D MRI data from a region of interest using a respiration gate technique. More specifically, a plurality of respiratory gate windows are set, and a pilot scan defining a plurality of corresponding slabs is performed. Then, during each of the plurality of respiratory gate windows, a scan is performed to acquire three-dimensional MRI data for a corresponding one of the plurality of slabs. The three-dimensional MRI data acquired for each slab is combined to form a single three-dimensional MRI data set representing the region of interest. Since MRI data is acquired over most of each respiratory cycle, it is possible to acquire the entire three-dimensional MRI data set in a relatively short scan time.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1, the major components of a preferred MRI system incorporating the present invention are shown. The operation of the system is controlled by an operator console 100, which includes a keyboard and control panel 102 and a display 104. Console 100 is connected to independent computer system 107 via link 116
And the computer system 107 allows the operator to control the formation and display of images on the screen 104. Computer system 107 includes several modules that communicate with each other via a backplane. These modules include an image processor module 106, a CPU module 108, and a memory module 113 known in the art as a frame buffer for storing image data arrays. The computer system 107 includes a disk storage device 111 and a tape drive 11 for storing image data and programs.
2 and a high-speed serial link 11
5 and a separate system control 122.

[0011] The system controller 122 includes a set of modules connected together by a backplane 118. These modules include a CPU module 119 and a pulse generator module 121, which is connected to the operator console 100 via a serial link 125. The system control unit 122 is connected via the link 125.
Receives commands from the operator indicating the scanning sequence to be executed.

The pulse generator module 121 operates the components of the system to execute a desired scan sequence. The pulse generator module 121 generates data indicating the timing, magnitude and shape of the RF pulse to be generated, and the timing and length of the data acquisition window. The pulse generator module 121 is connected to a set of gradient amplifiers 127 and indicates the timing and shape of the gradient pulses generated during a scan.
The pulse generator module 121 also receives patient data from the physiological acquisition controller 129, which receives signals from sensors connected to the patient. One such signal is an ECG (electrocardiogram) signal, which is processed by controller 129 to generate a heartbeat trigger signal for pulse generator module 121. The pulse generator module 121 is also connected to a scan room interface circuit 133, which receives signals from various sensors related to the condition of the patient and the magnet system. Via scan room interface circuit 133, patient positioning system 134 also receives instructions to move the patient to the desired position for the scan.

[0013] The gradient waveforms produced by the pulse generator module 121 are applied to a gradient amplifier system 127 comprised a G _x amplifier G _y and G _z amplifiers. Each gradient amplifier is generally designated by reference numeral 13
The corresponding gradient coils in the assembly shown at 9 are excited to generate the magnetic field gradient used to position encode the acquired signal. The gradient coil assembly 139 includes a polarizing magnet 140 and a whole-body RF coil 152.
And a part of the magnet assembly 141 including: Transceiver module 1 in system control unit 122
50 generate pulses which are amplified by an RF amplifier 151 and coupled to an RF coil 152 by a transmit / receive (T / R) switch 154. As a result, the signal generated by the excited nucleus in the patient's body is sensed by the same RF coil 152 and transmitted through the transmit / receive switch 154 to the preamplifier 15.
3 The amplified NMR signal is transmitted to the transceiver 1
Demodulated and filtered at 50 receiver sections,
Digitized. The transmission / reception switch 154 is controlled by a signal from the pulse generator module 121, and switches the RF amplifier 151 to the coil 15 in the transmission mode.
2 and a preamplifier 15 in the reception mode.
3 is connected. The transmit / receive switch 154 also provides separate RF for either transmit or receive modes.
Allows the use of coils (eg, head coils or surface coils).

NM detected by RF coil 152
The R signal is down-converted by the RF reference signal and then digitized by the transceiver module 150. The digitized NMR signal is transferred to the memory module 160 in the system control unit 122.
When the scan is completed and the entire k-space NMR data array has been acquired in the memory module 160, the array processor 161 operates to Fourier transform this data into an image data array. This image data is transmitted to the computer system 107 via the serial link 115 where the disk memory 111
Is stored. In response to a command received from the operator console 100, this image data is stored in the tape drive 1.
12 or may be further processed by image processor 106 and transmitted to operator console 100 for display on display device 104. Transceiver 150
For a more detailed description of US Pat.
52,877 and 4,992,736.

[0015] The MRI system of FIG.
Run the sequence and collect enough NMR data to reconstruct the desired image. With specific reference to FIG. 2, an exemplary three-dimensional gradient recalled echo pulse sequence is implemented wherein a G _z slab selective gradient pulse 301 is applied to a test object in the presence of a gradient pulse 301. Transverse magnetization is generated in the selected slab using the RF excitation pulse 300. The resulting N
In order to compensate the MR signal 303 for phase shifts caused by the slab selection gradient pulse 301 and to reduce the sensitivity of the NMR signal 303 to velocity along the z-axis, as taught in US Pat. No. 4,731,583. Next, a negative _Gz gradient pulse 304 followed by a positive _Gz gradient pulse 305 is generated by the _Gz gradient coil.
The gradient pulse 304 has many amplitudes and also performs phase encoding along the z-axis direction. Pulses 304 and 305
Compensates for velocity along the z-axis, but more complex gradient waveforms that compensate for acceleration and higher order motion are also well known to those skilled in the art.

To position encode the NMR signal 303, shortly after the application of the RF excitation pulse 300,
A phase encoding _Gy gradient pulse 306 is applied to the test object. As is well known in the art, a complete scan consists of a sequence of these pulse sequences, in which the value of the G _y phase encoding pulse is a series of 256 discrete phase encoding pulses, for example. Stepping through the values localizes the position of the spins that generate the NMR signal along the y-axis. The position along the x axis is NM
In order to frequency encode the R signal 303, the NMR gradient echo signal 303 is located by a G _x gradient pulse 307 generated at the same time as the acquisition. The gradient pulse 307 is preceded by a gradient pulse 307, as taught in U.S. Pat. No. 4,731,583, to generate a gradient echo 303 and to reduce the sensitivity of the echo signal 303 to velocity along the x-direction. Then, gradient pulses 308 and 309 are applied.

The NMR signal 303 is transmitted to the system transceiver 1
22 and digitized as a row of N _x (eg, 256) complex numbers and stored in memory. An NMR signal 303 is generated, acquired, digitized, and stored as a separate row of N _x (eg, 256) complex numbers for each combination of (G _y , G _z ) phase encoding gradients. You. Therefore,
At the completion of scanning, three-dimensional (N _x × N _y)
× N _z ) The array is stored. Where N _y is the number of phase encode steps along the y direction and N _z is z
Number of phase encode steps along the direction. Using this array of k-space data, an image can be reconstructed as described above.

It will be apparent to those skilled in the art that many other NMR imaging pulse sequences can be used. The imaging pulse sequence of FIG. 2 is suitable for heart rate imaging, which is a preferred application of the present invention. As described below, during image data acquisition, a navigator echo signal is also acquired to measure the displacement of the motion of the test object during scanning.

In one embodiment of the present invention, a conventional navigator pulse sequence is used to determine the position of the patient's diaphragm during each cardiac cycle. This navigator pulse sequence uses two-dimensional RF excitation pulses to
It excites a column of spins across the diaphragm, located on the right side of the abdomen and near the convex (dome) of the liver. NMR in the presence of a readout gradient (G _{z in the} preferred embodiment) oriented along the length dimension of the excited column
The signal is acquired and N _{ec ho} (eg, 256) samples of the NMR navigator signal are _{stored in the} array processor 161.
Is Fourier-transformed. The two-dimensional excitation RF pulse is, for example, a 30 mm diameter excitation that produces a flip angle of 90 °, but may excite other diameters. For example, as described in US Pat. No. 4,812,760, two-dimensional RF pulses such as these are generated in the presence of two gradient fields (G _x and G _{y in the} preferred embodiment). The low pass filter of the receiver is set to match the field of view (eg, 260 mm) along the excited column (z-axis). The NMR signal is sampled at N _echo points over a sampling time of, for example, 4 milliseconds. A reference navigator echo is acquired prior to acquiring the image data. Standard Navigator
Echoes are usually acquired at the end of exhalation because respiratory movements are relatively stable and reproducible in this position. The displacement between the current diaphragm position and the reference diaphragm position is determined by Magn. Reson. Med.
36, pp. 117-123 (1996).
An algorithm for extracting motion information from navigator echoes (Algorithms for Extracting Mo
Measurement Information from Navigator Echoes) can be measured using an autocorrelation algorithm and a least mean square algorithm. In addition, the diaphragm position is determined by Thom, filed on November 26, 1997.
It can also be measured by using the linear phase shift algorithm disclosed in US patent application Ser. No. 08 / 980,192 to Kwok-Fah Foo and Kevin F. King.

This conventional navigator pulse sequence is used for a heart rate gated scan as shown in FIG.
The QRS complex 320 of the ECG signal is
The start of the R interval is shown, during which segment 322 of NMR image data is acquired using the imaging pulse sequence of FIG. As is well known in the art,
Each segment 322 is a sampling of multiple lines in k-space from a single slab that passes through the patient's heart, and the acquisition is sufficient to reconstruct one or more images. Continue until is obtained.

In each heart cycle, reference numeral 324
A navigator pulse sequence as shown in FIG. 7 is executed to determine the position of the patient's diaphragm just prior to the acquisition of each segment 322. The navigator NMR signal is
It provides an indication of the diaphragm position immediately prior to each acquisition of image data during a heart rate gated MRI scan. As shown in FIG. 4, this diaphragm position information provides a respiratory signal 350 that circulates between one value at the end of the patient's exhalation and another value at the end of the patient's inspiration. As described in more detail below, the respiration signal 350 provided by the interleaved navigator pulse sequence 324 is used to gate image data acquisition from a plurality of three-dimensional slabs.

Referring specifically to FIG. 5, a scout MRI scan is performed to obtain a cross-sectional image of the patient's heart, indicated at 360. Coronary artery 362 is located on the surface of the heart. 3D space of interest is console 1
00 is defined by the operator using an interactive tool provided on the scout image.
The dimensional slab 364 can be arranged. In the example shown in FIG. 5, four overlapping slabs 1 to 4 have been specified by the operator and are input along with other scan parameters that define the three-dimensional heart rate gated MRI scan to be performed. In addition, the respiration signal 350, also acquired during the scout scan, is displayed on the console 100 and the operator sets a respiration gate window. In the example shown in FIG. 4, four separate gate windows 1 to 4 are set, and these windows 1 to 4 are assigned to four corresponding slabs 1 to 4. Each of the slabs is associated with a respiration gate window. One of the preferred correspondences between the breathing window and the slab is as follows. That is, window 1 corresponds to slab 1, window 2 corresponds to slab 2, window 3 corresponds to slab 3, and window 4 corresponds to slab 4. Another preferred correspondence between the breathing window and the slab is as follows. That is, window 1 corresponds to slab 4, window 2 corresponds to slab 3, window 3 corresponds to slab 2, and window 4
Corresponds to slab 1.

After a scout scan has been performed and the respiration gate window has been defined and assigned to the corresponding predefined slab, the scan begins. Referring to FIG. 3, during each cardiac cycle, navigator pulse sequence 3
24 is executed to calculate the value of the respiration signal. If the measured respiration position is located within one of the defined respiration gate windows, the image data segment 322 for the corresponding defined slab assigned to this window. Is obtained. If the respiration signal is not located within any of the defined respiration gate windows, no image data is acquired during this cardiac cycle.

The scanning continues until a three-dimensional image data set has been acquired for all of the defined slabs. However, scanning over all slabs requires much less time since four separate breathing gate windows are used and image data can be acquired over a much larger portion of the respiratory cycle. . Ideally, the size of the breathing gate window is set such that acquisition of each slab is completed at substantially the same time. To achieve this, the breathing gate window may be changed dynamically. Also, the slabs may be located in the axial direction or in any other orthogonal or oblique orientation.

After the slab data sets have been obtained, the data sets are concatenated to form a single three-dimensional data set for the entire region of interest. This can be performed in several ways, but in each case, the relative position of each acquired slab is adjusted to account for the movement that occurs between different breathing gate windows. Must have been.

One method of joining separate slabs involves manually aligning each slab. In the first stage, a three-dimensional slab image is reconstructed from each three-dimensional slab k-space data set by executing a three-dimensional fast Fourier transform as described above. The separate reconstructed slab images are then displayed on console 100 and manually moved to align these slab images with respect to each other using a trackball provided on control panel 102. Can be done. This alignment is possible because each slab 364 overlaps as shown in FIG. 5 and therefore includes a portion of the same anatomy, which is displayed on each of the adjacent slabs. It will be apparent that this registration step can also be performed automatically using well-known image registration methods.

Another method of connecting separate slabs is to shift the positions of these slabs by a measured amount. For each window 1-4 shown in FIG. 4, there is a corresponding spatial location of the patient's heart shown in FIG. That is, the heart is circulating as a result of breathing. During a scout scan, the position of the reference point on the heart can be measured as a function of the respiratory signal 350. A reference position is selected and the shift of this reference point in a direction away from the reference position at the center of each breathing gate window is calculated. Upon coupling, each slab is shifted in position by an amount corresponding to the shift of its breathing gate window. In this way, each slab is aligned with respect to the reference position.

These imaging slabs are defined and acquired at different positions with different orientations. For example,
One of the slabs can be positioned to acquire an image of the right coronary artery (RCA) and one of the other slabs can be positioned to acquire an image of the left anterior descending artery. In this example, no slab overlap is required and these slabs can be displayed separately.

[Brief description of the drawings]

FIG. 1 is a block diagram of an MRI system employing the present invention.

FIG. 2 is an example three-dimensional NMR imaging pulse sequence diagram that can be used in practicing the present invention.

FIG. 3 is a graph of a heart rate gated MRI scan using a navigator pulse sequence for respiratory gating in accordance with the present invention.

FIG. 4 is a graph of a respiratory signal showing the motion of the test object as a function of the patient's breathing.

FIG. 5 is a perspective view of a patient's heart showing how a three-dimensional space is scanned in four separate slabs.

[Explanation of symbols]

100 Operator Console 102 Keyboard and Control Panel 104 Indicator 111 Disk Storage 112 Tape Drive 115 High Speed Serial Link 116 Link 118 Backplane 125 Serial Link 139 Gradient Coil Assembly 140 Magnet for Polarization 141 Magnet Assembly 152 Whole Body RF coil 300 selective RF excitation pulse 301 G _z slab select gradient pulse 303 NMR signals 304 and 305 G _z gradient pulse 306 phase-encoding G _y gradient pulse 307 G _x gradient pulse 308, 309 gradient pulse 320 QRS Complexe 322 NMR image data Segment 324 navigator pulse sequence 350 respiratory signal 360 patient's heart 362 coronary artery 364 three-dimensional slab

Claims (10)

[Claims] 1. A method for acquiring a three-dimensional image of an examination object by a magnetic resonance imaging system, comprising: (a) setting a plurality of respiratory gate windows; and (b) three-dimensional images including the examination object. Setting a plurality of slabs by dividing a region of interest into a plurality of three-dimensional slabs respectively associated with each of the plurality of respiratory gate windows; and (c) detecting a respiratory position of the patient; Determining whether the detected respiratory position is within a window of each of the plurality of respiratory gate windows; and (d) determining that the detected respiratory position is within a single respiratory gate window. Acquiring nuclear magnetic resonance data from the associated three-dimensional slab with the magnetic resonance imaging system when located at: The method sufficient NMR data to construct has a a step of repeating steps (c) and (d) until the acquired for each slab. 2. The step (c) comprises: (a) executing a navigator pulse sequence with the magnetic resonance imaging system to generate a respiratory signal; and (b) the respiratory signal comprises the respiratory gate. Determining whether they fall within the range of values for one of the windows. 3. The method according to claim 1, wherein the object to be examined is a coronary artery, the method comprising: (f) generating a heart rate gating signal indicative of a phase of the patient's heart during each heart cycle. The method of claim 1, wherein (d) is performed at a selected cardiac phase indicated by the heart rate gating signal. 4. The step (c) comprises: (a) executing a navigator pulse sequence by the magnetic resonance imaging system during each cardiac cycle to generate a respiratory signal; Determining whether a respiratory signal falls within a range of values for one of the respiratory gate windows. 5. The step (b) comprises: (a) performing a scout scan with the magnetic resonance imaging system to obtain nuclear magnetic resonance data from the patient; and (b) the acquired nucleus. Reconstructing the image of the inspection target from magnetic resonance data; (c) showing the plurality of three-dimensional slabs on the image; (d) nuclear magnetic resonance from each of the indicated three-dimensional slabs Defining a pulse sequence for acquiring data;
The method of claim 1 comprising: 6. The method of claim 1, wherein the plurality of slabs set in step (b) are oriented at a plurality of different angles. 7. A magnetic resonance imaging system for forming a three-dimensional image of an anatomical structure to be examined in a patient's body, wherein each of the plurality of respiratory gate windows indicates a range of respiratory movement of the patient. And pulse generation for instructing the magnetic resonance imaging system to execute a pulse sequence for acquiring nuclear magnetic resonance data from each of a plurality of three-dimensional slabs in the anatomical structure under examination. A plurality of three-dimensional detectors when each of the respiratory gate windows is detected.
A pulse generator operable to acquire nuclear magnetic resonance data from a corresponding one of the three-dimensional slabs; connecting the nuclear magnetic resonance data acquired for each of the three-dimensional slabs; Means for reconstructing a three-dimensional image of the anatomical structure. 8. The anatomical structure to be examined is the heart of the patient, and the system detects beats of the patient's heart and determines the pulse sequence at a predetermined phase of the cardiac cycle. 8. A magnetic resonance imaging system according to claim 7, including heart rate gating means for triggering the pulse generator to perform the following. 9. The means for detecting the respiratory gate window comprises: commanding the pulse generator to obtain magnetic resonance data from the patient indicative of movement caused by respiration of the patient. 8. The magnetic resonance imaging system according to claim 7, comprising means for executing the sequence periodically. 10. The magnetic resonance imaging system according to claim 7, wherein the three-dimensional slab for acquiring the nuclear magnetic resonance data is oriented at a plurality of different angles.